/Engine/Source/Editor/LevelEditor/Public/ILevelEditor.h
After spawning a new level viewport outside of the editor's tab system, this function must be called so that the editor can keep track of that viewport
void AddStandaloneLevelViewport ( const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SLevelViewport](API\Editor\LevelEditor\SLevelViewport) > & LevelViewport )